breeding horses
Artificial insemination (AI) with cooled shipped semen is one of the most common methods for breeding in most countries. But studies have shown that this technique can result in lower pregnancy and foaling rates than live cover or AI with fresh semen.

“Understanding the most important factors that lead to lowered fertility is of significant interest,” said Katila Terttu, DVM, MS, PhD, professor emerita of animal reproduction at the University of Helsinki, in Finland.

In a study of 1,098 mares bred with chilled shipped semen from 90 stallions, she and colleagues identified three factors that can help mare owners decide when to order semen and inseminate.

1. Give the mare ample opportunities to become pregnant.

“The mare needs to get enough opportunities to be bred,” Terttu said. “That’s at least three cycles during the breeding season. Start to breed early, with pregnancy checks around Day 16-17 so that no heats are missed if the mare did not conceive.”

Working closely with a veterinarian is key, especially for mares with inconsistent cycles or silent heats. And mares’ fertility at the end of the breeding season tends to be good, she said. So she encouraged breeders to consider inseminating mares “once more” at the end of the season in breeds that don’t benefit from an early foal birthdate.

2. Breed as close to ovulation as possible.

This can help increase pregnancy rates because cooled sperm do not survive in the mare as long as fresh sperm does.

“More inseminations during the estrus gave better results,” Terttu said. “The best explanation for this is that the last insemination occurred close to ovulation. Use of ovulation-inducing agents to time the insemination close to ovulation is recommended.”

3. Select stallions with sperm that has good progressive motility.

In the team’s study, they found that the better the sperm’s progressive motility—that is, how well it moves in the uterus—in a dose of semen, the higher the conception rate. And, although motility has limitations and is not a perfect measure, it is the only one veterinarians can use to evaluate semen quality in practice, Terttu said.

“The number of progressively motile sperm at the time of insemination was significant; this emphasizes the ability of sperm to maintain motility during shipment,” she said. “Additionally, the lower the semen quality, the more important it is to inseminate close to ovulation.”

The Bottom Line

The team concluded that breeding mares multiple times in a season, inseminating close to ovulation, and choosing sperm with good progressive motility can help improve pregnancy and foaling rates. Work with your veterinarian to find good-quality semen for your mare and determine the ideal times to breed for the best chances of a live foal.

The study, “Analysis of factors affecting the pregnancy rate of mares after inseminations with cooled transported stallion semen,” was published in Theriogenology.